Re: [PATCH RFC v2] drm/cirrus-qemu: Validate BAR0 size during probe

[Slawomir Stepien <sst@poczta.fm>]

#syz upstream

On sie 24, 2026 13:12, syzbot wrote:
> The `cirrus-qemu` driver relies on `CIRRUS_VRAM_SIZE` (4 MB) to validate
> framebuffer sizes. However, during PCI probe, the driver mapped BAR0
> without verifying that its size matches `CIRRUS_VRAM_SIZE`.
> 
> If a PCI device with a BAR0 smaller than 4 MB is bound to the driver, the
> mapped VRAM will be smaller than expected. Because validation checks assume
> 4 MB VRAM, framebuffers larger than the mapped memory can be created.
> 
> When the display plane is updated (e.g. during release),
> `cirrus_primary_plane_helper_atomic_update()` copies the framebuffer to
> VRAM using `drm_fb_memcpy()`. Writing past the end of the mapped I/O memory
> causes a supervisor write page fault:
> 
> BUG: unable to handle page fault for address: ffffc9000389c000
> #PF: supervisor write access in kernel mode
> #PF: error_code(0x0002) - not-present page
> ...
> RIP: 0010:memcpy_toio+0x7c/0xe0 arch/x86/lib/iomem.c:110
> ...
> Call Trace:
>  <TASK>
>  iosys_map_memcpy_to include/linux/iosys-map.h:285 [inline]
>  drm_fb_memcpy+0x325/0x5d0 drivers/gpu/drm/drm_format_helper.c:442
>  cirrus_primary_plane_helper_atomic_update+0x98a/0xb00
>  drivers/gpu/drm/tiny/cirrus-qemu.c:358
>  drm_atomic_helper_commit_planes+0x626/0xea0
>  drivers/gpu/drm/drm_atomic_helper.c:3038
>  drm_atomic_helper_commit_tail+0x60/0x510
>  drivers/gpu/drm/drm_atomic_helper.c:1989
>  commit_tail+0x2b1/0x3c0 drivers/gpu/drm/drm_atomic_helper.c:2074
>  drm_atomic_helper_commit+0xa77/0xb10
>  drivers/gpu/drm/drm_atomic_helper.c:2312
> 
> Fix this by validating in `cirrus_pci_probe()` that the PCI BAR0 resource
> length equals `CIRRUS_VRAM_SIZE`, returning `-ENODEV` if it does not match.
> 
> Fixes: ab3e023b1b4c ("drm/cirrus: rewrite and modernize driver.")
> Assisted-by: Gemini:gemini-3.6-flash Gemini:gemini-3.1-pro-preview syzbot
> Reported-by: syzbot+2442951a6abb004df963@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=2442951a6abb004df963
> Link: https://syzkaller.appspot.com/ai_job?id=ba262a3a-bccf-4ad8-a1b0-583c55d34fd6
> To: "David Airlie" <airlied@gmail.com>
> To: "Dave Airlie" <airlied@redhat.com>
> To: <dri-devel@lists.freedesktop.org>
> To: "Gerd Hoffmann" <kraxel@redhat.com>
> To: "Maarten Lankhorst" <maarten.lankhorst@linux.intel.com>
> To: "Maxime Ripard" <mripard@kernel.org>
> To: "Simona Vetter" <simona@ffwll.ch>
> To: "Thomas Zimmermann" <tzimmermann@suse.de>
> To: <virtualization@lists.linux.dev>
> Cc: "Jani Nikula" <jani.nikula@intel.com>
> Cc: <linux-kernel@vger.kernel.org>
> 
> ---
> v2:
> - Reject device in probe if PCI BAR0 resource length does not equal CIRRUS_VRAM_SIZE instead of dynamically tracking VRAM size.
> - Retain CIRRUS_VRAM_SIZE and keep atomic check and mode validation hooks unchanged.
> 
> v1:
> https://lore.kernel.org/all/6e0e7e49-c50d-472e-9cc2-ceaf814e0fc7@mail.kernel.org/T/
> ---
> diff --git a/drivers/gpu/drm/tiny/cirrus-qemu.c b/drivers/gpu/drm/tiny/cirrus-qemu.c
> index 075221b43..70c995025 100644
> --- a/drivers/gpu/drm/tiny/cirrus-qemu.c
> +++ b/drivers/gpu/drm/tiny/cirrus-qemu.c
> @@ -602,6 +602,9 @@ static int cirrus_pci_probe(struct pci_dev *pdev,
>  
>  	dev = &cirrus->dev;
>  
> +	if (pci_resource_len(pdev, 0) != CIRRUS_VRAM_SIZE)
> +		return -ENODEV;
> +
>  	cirrus->vram = devm_ioremap(&pdev->dev, pci_resource_start(pdev, 0),
>  				    pci_resource_len(pdev, 0));
>  	if (cirrus->vram == NULL)
> 
> 
> base-commit: db2ddb87143519e20a95aa36c60b36107b736a58

-- 
Slawomir Stepien